1. Field of the Invention
This invention relates to a method of manufacturing electronic devices. More particularly, it relates to a method of manufacturing high voltage Schottky diamond diodes by growing pure epitaxial diamond layers of low boron doping using microwave plasma chemical vapor deposition.
2. Description of the Prior Art
Diamond films grown using chemical-vapor deposition (CVD) have attracted considerable interest due to their large number of technological applications ranging from flat panel displays for computers to electro-optical components. Such films are grown from a mixture of hydrocarbon gas, such as methane, and hydrogen that is broken by a hot-tungsten filament or microwaves to hydrocarbon radicals and atomic hydrogen.
Diamond having a larger band gap (5.5 eV) compared to SiC (3.4 eV), Ga (3.2 eV), and Si (1.1 eV) should enable the fabrication of diodes with larger breakdown voltages than exhibited by diodes formed in other semiconductors. However, diamond diodes and transistors have not exhibited breakdown voltages in excess of 500 V in any known published report, while diodes formed in other semiconductors exhibit significantly higher breakdown. The primary cause for this phenomenon of low breakdown voltage was observed to be the inability to control impurities in the diamond.
Diamond, having the highest atomic density of any terrestrial material, cannot easily incorporate other larger atoms into its crystal. The very few exceptions to this are the small atoms of hydrogen, boron, and nitrogen. Hydrogen is not known to substantially effect the electrical properties of single crystal diamond, but is known to bind with other impurities like boron making them electrically inactive. Boron is a p-type dopant with an activation energy of 0.36 eV, and nitrogen is a deep n-type dopant with an ionization energy of about 1.7 eV. Other impurities such as, for example, phosphorous, lithium, and sulfur are possible n-type dopants, but the results reported so far have been inconsistent.
In forming a diamond film through microwave plasma CVD, a boron-doped (B-doped) p-type diamond film is obtained by doping with boron, i.e., a metallic element of group III. B-doped diamond films are expected to form Schottky diodes having excellent heat and radiation resistance, properties not reported to have been achieved by conventional Schottky diodes produced with Si and GaAs. Other electronic devices having excellent characteristics may be fabricated using diamond films.
Methods which are commonly used to grow thin films of semiconducting diamonds include microwave plasma assisted CVD techniques and a hot filament assisted CVD techniques. In these methods, feed gases such as CH4, CO, and H2 are decomposed using a microwave induced plasma or a hot filament. This results in the formation of thin diamond films on a heated substrate made of Si, Mo, diamond, etc. Addition of B2H6 to the feed gases allows thin films of p-type diamond to be obtained.
FIG. 1 shows a prior art Schottky diode formed by depositing a heavily B-doped p-type polycrystalline diamond film on a p-type Si substrate b (the diamond film was prepared using diborane (B2H6) and a 5% CO gas diluted by H2). An In electrode c, which has an ohmic characteristic, is placed on the back side of the Si substrate. An Al needle electrode is placed on the lightly B-dope p-type polycrystalline diamond film a. This point contact electrode has Schottky characteristics. As shown in FIG. 2, this Schottky diode prevents a flow of electric current between electrodes c and d when a positive voltage is applied to the Al needle electrode d relative to the In electrode c, whereas it permits a flow of electric current, when a negative voltage is applied to the Al needle electrode.
A study of a Schottky diode similar to that shown in FIG. 3 has been reported in Hicks et al., xe2x80x9cThe barrier height of Schottky diodes with a chemical-vapor-deposited diamond base,xe2x80x9d J. Appl. Phys., 65(5), 2139-2141 (1989). This Schottky diode is formed by depositing a polycrystalline diamond film on a p-type (100) Si substrate bxe2x80x2 having a resistivity in the range of 0.01 to 0.1 ohm-cm (the diamond film was prepared by microwave plasma CVD process using CH4 diluted by H2 as a source gas). An ohmic contact electrode cxe2x80x2 is attached on the backside of the substrate, and an Au electrode dxe2x80x2 of a diameter of 0.1 cm and a thickness in the range of 140 to 500 Angstroms is deposited on the polycrystalline diamond film axe2x80x2. The corresponding I-V characteristics are shown in FIG. 4.
In order to fabricate devices which use semiconducting diamonds, it is necessary to provide thin films of high-crystalline quality with few dislocations and point defects. Conventionally, these films are formed by heating the substrate to a suitable temperature under a suitable degree of vacuum and introducing the feed gases immediately to grow a thin film.
FIG. 5 illustrates a conventional process of forming thin diamond films. X-axis plots time and Y-axis plots temperature. Two successive steps are differentiated by a vertical line. The gases to be introduced are indicated in the space defined by the temperature line and the partition lines. After a suitable vacuum is drawn, H2 gas is introduced into the vacuum chamber and the temperature is raised. When the substrate has been heated to an appropriate growth temperature, feed gases, such as, for example, CH4, H2, and optionally B2H6, are introduced. B2H6 is added to the reaction when a p-type crystal is to be formed. The introduced gases are excited by microwaves, heat, a high-frequency plasma, and other excitation methods, thus causing the feed gas to enter into a vapor phase reaction to form a thin film of diamond on the substrate.
However, thin films obtained by the use of conventional methods of vapor-phase synthesis contain a large number of defects and impurities. Unintentionally doped p-type films generally result from residual boron contamination in the reactor, or contain high concentrations of electronic trap states within the bandgap of diamond from impurity incorporation, usually due to nitrogen. The low quality of such films makes it impossible to fabricate devices that make use of the inherently good physical properties of diamond. This may be due to the fact that the impurities or defects remain on the surface of the diamond substrate.
Exceptionally pure epitaxial diamond layers have been grown by microwave plasma chemical vapor deposition having low boron doping, from 5xc3x971014 to 1xc3x971016 cmxe2x88x923. The compensating n-type impurities are the lowest ( less than 3xc3x971013 cmxe2x88x923) reported for any semiconductor diamond. The hydrogen impurities that bind with the boron making them electrically inactive appear to have been reduced by baking the diamond to  greater than 700xc2x0 C. for xcx9c1 s in air. Schottky diodes made on these epitaxial diamond films have breakdown voltages  greater than 6 kV, which is about 12 times the highest known, reported breakdown voltage for any diamond diode and higher than the breakdown voltage than any other known, reported semiconductor Schottky diode of similar thickness.
The invention comprises a method of making a Schottky diode comprising the steps of: providing a single crystal diamond comprising a surface; placing the single crystal diamond in a CVD system; heating the diamond to a temperature of at least about 950xc2x0 C.; providing a gas mixture capable of growing diamond film and comprising a sulfur compound through the CVD system; growing an epitaxial diamond film on the surface of the single crystal diamond; baking the diamond at a temperature of at least about 650xc2x0 C. in air for a period of time that minimizes oxidation of the diamond; and fabricating a Schottky diode comprising the diamond film.
The invention further comprises a Schottky diode comprising an epitaxial diamond film and capable of blocking at least about 6 kV in a distance of no more than about 300 xcexcm.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.